(A) Protein biosynthesis on a chip and in solution. (A) Protein biosynthesis on a chip and in solution. Transcription/translation of luciferase using the T7/E. Coli‐extract system in solution and on the chip with identical number of genes and reaction volume; the arrows indicate the position and direction of the T7 promoter within the 2160‐bp‐long dsDNA. The total amount of luciferase synthesized after 3 h of incubation is plotted. (B) Expression from top‐ and bottom‐oriented genes. Chips with varying amounts of DNA, and hence density, were prepared with genes oriented either with the promoter directed into the surface (top) or toward the solution (bottom) sides. The amount of luciferase synthesized as a function of gene number is plotted (Material and methods). (C) Effect of brush structure on expression. Transcription/translation on a chip from variable ‘gene‐only’ (left bottom scheme) and fixed ‘gene/dummy’ density brushes (right bottom scheme). The ‘gene/dummy’ brushes were maximally packed at the highest density with varying ratio of gene‐to‐dummy DNA. The amount of expressed luciferase produced in a 10 μl reaction volume placed on top of a 7 mm2 surface covered with DNA is plotted as a function of gene number. (D) Effect of brush structure on expression utility. Transcription/translation on a chip from variable ‘gene‐only’ and fixed ‘gene/dummy’ density brushes. The expressed luciferase per gene is plotted as a function of mean intergene distance. (E) Transcription‐only configuration in a buffer on a chip as a function of gene number for ‘top’ and ‘bottom’ orientations. (F) Transcription‐only configuration in a buffer on a chip from variable ‘gene‐only’ and fixed ‘gene/dummy’ density brushes. Amnon Buxboim et al. Mol Syst Biol 2008;4:181 © as stated in the article, figure or figure legend